Field of the Invention
The invention lies in the semiconductor technology and manufacturing field. More specifically, the invention relates to a mask for projecting a structure pattern onto a semiconductor substrate in an exposure unit. The exposure unit has a minimum resolution limit defining a minimum attainable lateral extent for a structure pattern element which is to be projected onto the semiconductor substrate.
Structures are formed on a semiconductor substrate usually by means of projection of a structure pattern from a mask onto the semiconductor substrate. This is done via a lens system in an exposure unit. In order to ensure the quality of a mapping operation and the functionality of the integrated circuit which is to be produced, the process involves prescribing a tolerance by which the lateral extent of the structure formed on the semiconductor substrate may differ from that on the projected mask or design master taking into account a reduction factor for the mapping.
Free parameters available when carrying out a projection in an exposure unit are, by way of example, the exposure dose and the setting of a focus value for the lens system in question. Generally, what is obtained is a number of combinations for the two aforementioned exposure parameters, for which a difference, which can be measured using a microscope, for example, is smaller than the prescribed tolerance value. A two-dimensional range for the combinations which satisfy such a condition is also called a process window.
For a fixed exposure dose, a range of admissible focus values for the exposure is available as a section through this process window. This range, which is also referred to as the depth of field, ideally has a large extent. This is because, firstly, with a large number of preliminary processes, a complicated surface topography may already have been produced on the semiconductor substrate by forming a corresponding number of levels in a circuit. Accordingly, it must be possible to carry out mapping at various levels, but in one mapping operation, in each case with high definition.
Secondly, by way of example, lens aberrations over the image plane of the semiconductor substrate bring about a distribution of differences in the local focus value with respect to a mean value for the focus. The differences (defocus) cannot be corrected without adversely affecting the other regions in the image plane.
Modern techniques for improving the resolution for projection, “lithographic resolution enhancement techniques”, equally result in a disadvantageous reduction in the depth-of-field range if they are intended to be used to transfer structure elements in the neighborhood of the resolution limit of the exposure unit to the image plane. These techniques include, by way of example, the use of off-axis illumination (OAI) or the use of half-tone phase masks.
Since reducing the resolution limit is aimed particularly at projecting particularly dense structures, as in memory production, for example, the problem of the depth-of-field range being too small arises particularly for structure elements which are arranged on the substrate in isolated fashion. This is because lens aberrations and the respective illumination setting used, for example off-axis illumination or particular aperture shapes, have different effects on densely arranged and isolated structure elements.
As a solution, various other techniques have been proposed, among which, by way of example, the alternating phase masks can be mentioned which, on account of their properties, have only an insignificant influence on the depth-of-field range in relation to the isolated structures. However, they are very complex to produce and usually require double exposures, which signifies a considerable increase in the expense of the overall production process.
Another solution is to carry out double exposure for respective dense structures and for the isolated structure elements in the peripheral region. This results in an increased cost outlay and possibly in a reduction in quality on account of additional alignment to be carried out during exposure.
Another solution involves arranging “sublithographic structures” (subresolution assist features (SRAF)) in the immediate surroundings of the isolated structure elements on the mask. These sublithographic structure elements have a lateral extent which is smaller than the minimum lateral extent which can be attained with the exposure unit. They are thus not mapped on the semiconductor substrate under normal exposure conditions. Their proximity to the isolated structure means that the sublithographic structure element delivers a light and phase contribution to the mapping of the isolated structure, however, similarly to the way in which this would bring about adjacent structure elements within a dense structure configuration, for example. The sublithographic structure elements therefore simulate a dense structure configuration around the isolated structure element.
One drawback of this solution is that the allocation and dimensioning of these sublithographic structure elements as auxiliary structures for isolated structure elements require complicated computation methods to be carried out taking into account the circuit's design rules. The design rules include conditions which need to be used to match the position and distances of structure elements to one another over a plurality of circuit levels. Another drawback is the increased sensitivity to the “mask error enhancement factor” (MEF), which describes a nonlinear, very acutely increasing relationship between the difference in the lateral extents of structures on the mask and the order of magnitude of the difference in the lateral extent of the same structures on the semiconductor substrate actually in the region of the resolution limit, i.e. the minimum lateral extent which can be obtained by the exposure unit. Yet another drawback is the significant reduction in the resolution with which structures are produced on the mask in binary form. The small extent of the sublithographic structure elements means that this resolution needs to be chosen to be particularly small, which significantly increases the writing time and hence the cost outlay for producing the mask.